As a capacitance type pressure sensor of this kind, for example, as disclosed in Japanese Published Patent App. No. JP2009-300336A and shown in FIG. 3, a diaphragm is jointed by welding to a periphery of an opening of a recessed portion formed in one end of a sensor main unit to which a fixed electrode is fixed by a sealing glass.
Although not shown in Japanese Published Patent App. No. JP2009-300336A, as shown in FIG. 3, a flange part, which is attached to a flow path forming member for forming a flow path to be measured so as to lead a fluid to a pressure receiving surface, is jointed by welding to a circumference of a side of the pressure receiving surface of the diaphragm that is welded to the sensor main unit. In the capacitance type pressure sensor as configured above, a fluid flows into a side of the diaphragm through an introduction port provided in the flange part so that the diaphragm is displaced by a pressure of the fluid.
However, in the capacitance type pressure sensor as constructed above, when the diaphragm is welded to the sensor main unit, there is a problem that, expansion amounts of the sensor main unit and the diaphragm are different because thermal expansion coefficients of the sensor main unit and the diaphragm are different and, therefore, there arises a deformation due to a thermal stress caused between the diaphragm and the sensor main unit after welding. Herein, although it may be considered to execute the welding taking into consideration the thermal expansion coefficients of both the sensor main unit and the diaphragm, the thermal expansion coefficient of the sensor main unit is not univocally defined since the sensor main unit is an assembly of the fixed electrode, sealing glass and the like, while the thermal expansion coefficient of the diaphragm is known because the diaphragm is made of a simple substance. Therefore, it is extremely difficult to execute the welding taking into consideration the thermal coefficients of both the sensor main unit and the diaphragm at the time of welding.
Further, similarly, in the case of welding the flange part to the diaphragm previously welded to the sensor main unit, there is a problem that there arises a deformation due to a thermal stress caused between the diaphragm and the flange part because the thermal expansion coefficients of the diaphragm and the flange part are different. Also, in this case, although it may be considered to execute the welding taking into consideration the thermal expansion coefficients of both the diaphragm and the flange part, the thermal expansion coefficient of the diaphragm welded to the sensor main unit is different from the thermal expansion coefficient of the diaphragm of the simple substance per se. Therefore, it is extremely difficult to execute the welding taking into consideration the thermal coefficients of both the diaphragm and the flange part at the time of welding.
In the above description, although the explanation is made as to the case where the flange part is welded to the diaphragm after the diaphragm is welded to the sensor main unit, there is a similar problem also in the case where the sensor main unit is welded to the diaphragm after the diaphragm is welded to the flange part.